Relationship between cation substitution and stability of perovskite structure in SrCoO3–δ-based mixed conductors

In SrCoO3–δ (SC)-based mixed conductors, cation substitution is necessary to stabilize the high-oxygen permeable perovskite structure, but the dose of the substitution should be minimized to avoid degradation of permeability. To clarify the relation between the substitutional cation and the perovskite stability of the mixed conductor, SC-based oxides, (La0.1Sr0.9)CoO3–δ and Sr(Co0.9X0.1)O3–δ where X was Ni, Cu, Zn, Cr, Fe, Al, Ga, In, Ce, Ti, Zr, Sn, V and Nb, were studied. The appearance of the low oxygen permeable 2H–BaNiO3-type SrCoO2.52 phase during the preparation of the powder samples or post-annealing in oxygen was investigated and the tendency to transform to the hexagonal phase was evaluated. The sequence of the perovskite stability upon the substituting cation for SC was{Ni, Cu, Zn, In, Ce} < {Cr, Al, Ga, Zr, Sn, V} < La < Fe < Ti < Nb.Thermogravimetry revealed that a rise in the valence of the substitutional cation increases the oxygen content, 3–δ, and enhances the stability of the perovskite structure. Oxygen permeability of the ceramic disk samples of SC, (La0.1Sr0.9)CoO3–δ (LaSC), Sr(Co0.9Fe0.1)O3–δ(SCFe), Sr(Co0.9Ti0.1)O3–δ (SCTi), Sr(Co0.9Nb0.1)O3–δ (SCNb) and Sr(Co0.8Fe0.2)O3–δ (SCF1082) were measured: the order of oxygen permeability at 900 °C wasSCNb ≥ SC > SCTi > SCFe > LaSC > SCF1082.Comparison of the sequence of the perovskite stability and oxygen permeability suggests that oxygen permeability is not in a trade-off relation against the stability of the perovskite phase. Within the measured samples, SCNb exhibited the highest stability of the perovskite structure, the highest oxygen permeability of 4.24 cm3/min/cm2 at 900 °C, good gas-tightness and steady thermal expansion of 22.8 ppm/°C. Niobium substitution on the Co-site was found for the first time to be effective in preparing an SrCoO3–δ based mixed conductor for application as an oxygen separation membrane.